Implantable medical devices (IMDs) for monitoring a physiological condition or delivering a therapy typically include one or more physiological sensors. Physiological sensors used in conjunction with an IMD provide a signal related to a physiological condition from which a patient state or the need for a therapy can be assessed. Examples of such IMDs include heart monitors, pacemakers, implantable cardioverter defibrillators (ICDs), myostimulators, neurological stimulators, drug delivery devices, insulin pumps, glucose monitors, etc.
Optical sensors are employed in IMDs as physiological sensors configured to detect changes in light modulation by a body fluid or tissue measurement volume due to a change in a physiological condition in the body fluid or tissue. Such optical sensors can be used, for example, for detecting changes in metabolite levels in the blood, such as oxygen saturation levels or glucose level, or changes in tissue perfusion. Monitoring such physiological conditions provides useful diagnostic measures and can be used in managing therapies for treating a medical condition. For example, a decrease in blood oxygen saturation or in tissue perfusion may be associated with insufficient cardiac output or respiratory function. Thus monitoring such signals allows an implantable medical device to respond to a decrease in oxygen saturation or tissue perfusion, for example by delivering electrical stimulation therapies to the heart to restore a normal hemodynamic function.